1. Field of the Invention
The present invention relates to a master information carrier for magnetic transfer, which is used in a magnetic transfer method for magnetically transferring information from the master information carrier, which carries the information to be transferred, to a slave medium, to which the information is transferred, and a method for producing the master information carrier for magnetic transfer
2. Description of the Related Art
In magnetic transfer, which is the subject of the present invention, a master information carrier (patterned master) has a magnetic layer at least as a surface layer. A transfer pattern such as a servo signal is formed in embossed shapes on the master information carrier. A magnetization pattern signal corresponding to the information carried by the master information carrier is transferred to and recorded on the slave medium by applying a magnetic field for transfer while the master information carrier is in close contact with the slave medium having a magnetic recording section.
In the case that the above-mentioned slave medium is a disk-shaped medium such as a hard disk or a high-density flexible disk, the aforementioned master information carrier is also disk-shaped and has a transfer pattern of concentric circles. A magnetic field for transfer is applied by arranging a magnetic field applying device such as an electromagnetic device or a permanent magnetic device at one or both sides of the master information carrier while the master information carrier is in close contact with a side or both sides of the slave medium.
As an example of the master information carrier used in the above-mentioned magnetic transfer, a master information carrier, wherein an embossed pattern corresponding to an information signal is formed on a surface of a substrate and a surface of the embossed pattern is coated with a thin magnetic layer, has been proposed (refer to U.S. Patent Laid-Open No. 20010028964, for example).
The master information carrier, which is the subject of the present invention, is produced through the following steps, for example. First, an original disk which has an embossed pattern made of a resist is obtained by coating a Si substrate with an electron beam resist or a photoresist. Then, a transfer pattern is drawn by exposing the electron beam resist or the photoresist to an electron beam, laser beam or the like after baking the electron beam resist or the photoresist. Thereafter, the electron beam resist or the photoresist is developed. Next, a conductive layer is applied on the embossed pattern on the original disk by sputtering, for example. Further, a metal disk (Ni electroforming layer) with a predetermined thickness is laminated on the conductive layer by electroforming Ni. Then, a master substrate (replica) is produced by peeling off the metal disk from the original disk and forming a disk in a predetermined size from the metal disk by die-cutting. Alternatively, the master substrate (replica) may be produced by using a master substrate before die-cutting as the original disk, carrying out electroforming repeatedly, peeling off a metal disk and forming a disk in a predetermined size from the metal disk by die-cutting. Then, a magnetic layer is deposited on a surface of the embossed pattern on the master substrate, and the master information carrier which has the embossed pattern formed on the magnetic layer is produced.
In the magnetic transfer using the above-mentioned master information carrier, a magnetic signal corresponding to the transfer pattern is transferred to and recorded on the slave medium by placing the master information carrier and the slave medium, such as a hard disk or a flexible disk, in close contact with each other and applying an external magnetic field for magnetic transfer.
The above-mentioned technique for producing a stamper by electroforming Ni is widely used in manufacturing optical disks or the like. Generally, disk substrates made of resin are produced from master substrates (stampers) by using an injection molding machine in manufacturing the optical disks. Therefore, some distortion (deformation) of the master substrate is removed by applying pressure at the time of injection molding. On the other hand, in the magnetic transfer, intervals of the embossed pattern are finer than those of the optical disks. A formation unit of the pattern in the magnetic transfer is 300 nm or less and as low as 50 nm or less, for example. Therefore, high accuracy is required.
It is essential that the master information carrier is in close contact with the slave medium evenly without any space between them to realize high-quality signal transfer in the above-mentioned magnetic transfer. Therefore, a contact pressure is increased or air is discharged by vacuum suction so that an air bubble is not formed on the contact surfaces that face each other.
However, if the contact pressure is increased, the pattern on the master information carrier may be destroyed or deformed and the durability properties of the master information carrier may be reduced. Therefore, it is impossible to increase the contact pressure to an extremely high level in the master information carrier. Since the master information carrier is expensive, high durability properties are required.
In the aforementioned master information carrier, wherein the master substrate includes the Si substrate, the amount of warp and distortion is small. However, the step of forming the pattern made of a magnetic material on the Si substrate is complex and time consuming. Further, there is a problem that the cost is high. On the other hand, in production of the master information carrier, wherein the master substrate is produced from the original disk by electroforming Ni or the like, and the master substrates are replicated from the produced master substrate, the master information carriers can be easily produced. Further, since a plurality of master substrates may be replicated from a single original disk, there is a cost advantage and it is practical.
However, in the above-mentioned master information carrier including the master substrate which has been produced by laminating and peeling off the metal disk using the original disk, the surface of the master substrate is not always even. The master substrate is deformed in the step of peeling off the metal disk from the original disk, the step of forming the disk in the predetermined size by die-cutting, etc. Consequently, the master substrate is warped or distorted.
If the master substrate, i.e., master information carrier, is warped or distorted, the contact properties of the master information carrier with the slave medium decreases, which causes spacing between the master information carrier and the slave medium. Particularly, since bit intervals are 300 nm or less, the amount of the above-mentioned spacing highly affects the transfer properties. Additionally, the surface properties of the master information carrier and the slave medium, the thickness of a protective layer which covers the magnetic layer, to improve the durability or the like, also affect the transfer properties. Therefore, it is basically important to reduce the warp and distortion of the master information carrier.
A temperature of solution at the time of electroforming, a method for changing electric currents to be applied, solution concentration, etc. affect deformation of the master information carrier which has been produced by electroforming Ni. However, these effects can be reduced by managing the production process. On the other hand, it is difficult to automate the step of peeling off the metal disk on the original disk, formed by electroforming and the accuracy is higher when the metal disk is manually peeled off. However, the metal disk which will become the master substrate tends to be distorted or deformed by the forces acting on the metal disk when the metal disk is peeled off.
Generally, there is thickness distribution in an electroformed product. The thickness varies in an area around an edge and the thickness distribution is relatively stable around a center. The thickness distribution also varies according to conditions such as the distance between a positive pole and a negative pole of an electroforming device, the design of a device, such as a parallel degree, the temperature of solution, density of electric currents and the type of solution.
Normally, a stamper for optical disk functions without any problem when the accuracy is approximately ±1 μ. However, since the master information carrier for magnetic transfer is required to be closely contacted with the slave medium, the master information carrier is required to be produced accurately so that a more even thickness is realized.